Fix black background in preview window

Problem found and fixed on GNU/Linux
wxWidgets 3.0 built with GTK3+ support and running on Xorg.

Avoid division by zero in case width or height for page are not specified

traceback:
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 3755, in OnPaint
self.DoDrawing(dc)
File "/home/developer/WorkData/PLC/beremiz/beremiz/editors/Viewer.py", line 3723, in DoDrawing
for y in xrange(self.PageSize[1] - (ystart * SCROLLBAR_UNIT) % self.PageSize[1], int(window_size[1] / self.ViewScale[1]), self.PageSize[1]):
<type 'exceptions.ZeroDivisionError'>: integer division or modulo by zero

IDManager: added import/export plus little cosmetic enhancements.

IDManager : refactored a bit, moved some code into PSKManagement.py. Now captures URI and PSK on new PYRO(S) and propose them when editing URI. Import/export still to be implemented.

Renaming servicetype into scheme.

Added toolbar button to launch ID Manager dialog.

Renamed IDManager control into IDBrowser, because dialog will be named IDManager

IDManager.py : finished selector mode.

Added early implementation of IDManager.py. For now only used to select ID in URIEditor

Path may contain spaces... sequel of d1359cb

'Merged' URI editor dialog and Discovery dialog, also clean up related code

Cut more useless code from URI Editor

URI Editor Dialog : moved it to /dialog, and fixed naming.

URI editor : Rewrote most of it, cleaner.

PSK : Stunnel and/or OpenSSL (undocumented) handles PSK better without special chars, apparently

Prevent PLCObject to be imported from IDE as a side effect of importing runtime package

WAMP : ensure that unicode string is passed for published events IDs

PYRO PSK: adjusted cipher and key length to be usable with openSSL 1.0.2k.

Automatically get PSK and ID when connecting to PYRO[S], so that future connection through PYROS can use that same key. Also fixed case to UPPER for *PSK.

Runtime : Ensure that a random PSK secret compatible with stunnel is generated if -s commandline switch is used. Stunnel service is restarted after generation, using spawn_subprocess. TODO : give stunnel restart command as a commandline parameter.

Added runtime/spawn_subprocess.py. Force use posix spawn instead of fork, with API similar to subprocess. Using fork in runtime is incompatible with Xenomai, because memory is locked and this can lead to out of memory error.

Small cosmetic change to enhance readability and avoid confusion.

Force Zeroconf to listen and answer only on interface specified with '-i' command line argument.

Useless __name__ == '__main__' tests.

Fixed PYRO's PSK_Adapter : monkey patching was breaking non-PSK protocol, and import ssl was missing.

merge

Fix PYROPSK protocol configuration. After a few iteration of trial and error it appears that TSLv1 and PSK ciphers needs to be specified

merge

PYRO/TLSPSK : must use PYROLOC* protocol scheme in pyro URI, otherwise object ID is missing. Had to use more persuasive pyro3 monkey patching to have PYROLOCPSK resolved properly

PYRO/TLSPSK : fixed typos, used appropriate ciphers (https://github.com/drbild/sslpsk/issues/3), use PYROPSK instead of unresolvable PYROLOCPSK.